A guide to Fibre Channel technology

This chapter offers an overview of Fibre Channel specifications and devices including HBAs and switches. Readers will also learn about the pros and cons of FC SANs in the storage enterprise.

For demanding applications, Fibre Channel (FC) technology has entrenched itself as the quintessential storage area network (SAN) interconnect, providing high speed, high reliability, and inherent security for storage network users. Fibre Channel technology is a block-based networking approach based on ANSI standard X3.230-1994 (ISO 14165-1). It specifies the interconnections and signaling needed to establish a network "fabric" between servers, switches and storage subsystems such as disk arrays or tape libraries. FC can carry virtually any kind of traffic.

Fibre Channel uses optical fiber, coaxial copper or twisted pair copper cabling to carry SAN data at speeds of 1 Gbps, 2 Gbps, 4 Gbps and (more recently) 10 Gbps. At the same time, latency is kept very low, minimizing the delay between data requests and deliveries. For example, the latency across a typical FC switch is only a few microseconds. It is this combination of high speed and low latency that makes FC an ideal choice for time-sensitive or transactional processing environments. These attributes also support high scalability, allowing more storage systems and servers to be interconnected. Fibre Channel is also supports a variety of topologies, and is able to operate between two devices in a simple point-to-point mode, in an economical arbitrated loop to connect up to 126 devices, or (most commonly) in a powerful switched fabric providing simultaneous full-speed connections for many thousands of devices. Topologies and cable types can easily be mixed in the same SAN.

Fibre Channel technology denotes four main service "classes" to meet a variety of enterprise needs. FC Class-1 involves a dedicated connection running at full bandwidth using delivery confirmations. FC Class-2 still provides confirmed delivery, but does not use a dedicated connection. FC Class-3 does not confirm delivery, though this reduction in overhead can improve apparent performance slightly. FC Class-4 provides confirmed delivery along with advanced features such as virtual connections and fractional bandwidth.

Fibre Channel is regarded as a very reliable SAN technology. The host bus adapters (HBAs) and switches are generally quite robust, minimizing the rate of device failures. The FC SAN fabric allows for multiple connection paths and redundant connections, so if a hardware fault or cabling issue arises, a new path can be found and communication can failover to an alternate connection -- keeping storage and applications connected (even at reduced performance) until corrective action can be taken. Alternatively, multiple connections can be aggregated (or trunked) for even better bandwidth. For example, two 2 Gbps connections can be aggregated so that they effectively behave as one 4 Gbps connection. The availability of multiple or redundant connections enables load balancing where SAN traffic is analyzed and can be dynamically rerouted from busy paths (bottlenecks) through less-used paths.

Security is another important attribute of Fibre Channel technology. A "network" lets multiple devices communicate together. But for a SAN, it's generally not desirable to allow every server to recognize or access every LUN on the SAN. In actual practice, LUNs must be configured so that they are visible to only certain applications -- configuring security is a core part of the storage provisioning process. Zoning makes it possible for devices within a Fibre Channel network to see each other. By limiting the visibility of end devices, servers (hosts) can only see and access storage devices that are placed into the same zone. Once the SAN is zoned, LUNs are masked so that each host server can only see specific LUNs.

However, there are some recognized disadvantages to FC. Fibre Channel has been widely criticized for its expense and complexity. A specialized HBA card is needed for each server. Each HBA must then connect to corresponding port on a Fibre Channel switch -- creating the SAN "fabric." Every combination of HBA and switch port can cost thousands of dollars for the storage organization. This is the primary reason why many organizations connect only large, high-end storage systems to their SAN. Once LUNs are created in storage, they must be zoned and masked to ensure that they are only accessible to the proper servers or applications; often an onerous and error-prone procedure. These processes add complexity and costly management overhead to Fibre Channel SANs.

Check out the entire iSCSI vs. FC handbook.

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